The present invention relates generally to multi-taps, and more particularly to multi-taps that incorporate directional coupling, forward and/or reverse path equalization, signal filtering and rejection, and so forth.
In present cable television operating systems television and other signals, such as data signals, are transmitted from a main source known as the xe2x80x9cheadendxe2x80x9d to remote subscribers, such as households. In typical cable operating systems the signal communication is unidirectional or a one-way transmission from the headend to the subscriber. In such systems, the transmitted signal is typically generated from a single signal source, permitting cable operators to maintain high quality signals for reception by the subscribers through use of tight tolerances, high quality shielding, and forward equalization to maintain reliable transmission. Recent advances in cable television have allowed subscribers to become interactive, that is to transmit return signals, typically through use of a modem, back to the headend. As a result, many cable systems now provide an active return path, but the signal sources such as modems at each subscriber and electrical noise generated from each household return signals that contain noise, whereby the noise sources are additive from all subscribers generating return signals at a given time. It is very difficult in present systems using known means to maintain a high quality return path, in that the cable operator cannot maintain any meaningful control of the signal source generating the return signal from each subscriber""s home, for example.
The three primary sources of noise generated in the return path of a bidirectional cable television operating system are thermal noise, ingress, and fiber link noise. More specifically, thermal noise is generated by the various electronic components and active devices such as amplifiers. Typically thermal noise is caused by the thermal heating of resistive components.
Ingress noise can be generated from a number of different noise sources. Devices such as citizens band radios, short wave receivers, and amateur radio transmitters can all generate background radiation that causes noise in the return frequency band of the cable television system. Ingress noise can also be generated by broadband noise derived from switching transients, electrical arching, intermittent ground connections, and so forth. Ingress noise can also be generated by other electrical devices used in the home, and by unterminated cable television splitters, and incorrectly or improperly connected cables and connectors used in the household cable television signal distribution. Standard television receivers are not designed to include appropriate filters, and/or attenuators, for rejecting such noise in the frequency band of the return path.
Another source of ingress noise is fiber link noise. Such noise is generated in systems where a radio frequency signal is applied to a laser in the return path, producing an optical output that contains fiber link noise. The fiber link noise signal generated by one or more subscribers is carried through the fiber optical portion of a main cable 61 to a photo receiver in the headend, where such noise may cause unreliable communication.
The typical cable television system presently in use only provides forward path equalization. In such systems, the lack of reverse path equalization permits reverse path signals and/or ingress noise to overdrive amplifiers at the headend causing the output signals of the amplifiers to be non-linear and contain clipping distortion. The present inventor recognized that in order to reduce this problem, the return path must be equalized to provide an equal signal level on a return path from all subscribers, whereby in the worst case no over driving of amplifiers at the headend will occur.
It is known in the prior art to provide multi-tap devices with single plug-in modules that provide the functions of directional coupling and equalization. Accordingly, such prior multi-taps permit a user to select a single plug-in for providing a desired level of directional coupling, and some desired equalization function. Such prior multi-tap devices are an improvement over the use of multi-tap devices having hardwired tap plates, where the entire tap plate must be replaced in order to change the level of directional coupling, and/or equalization or filtering specifications. However, although an improvement over prior hardwired multi-tap devices, the inventory requirements for the single plug-in module multi-tap devices are very burdensome. For example, if a user wishes to have individual plug-in modules for selectively providing ten (10) different levels of directional coupling, thirty five (35) different levels of various equalization functions, and five (5) different levels of filtering, such as high-pass filtering, noise filtering, and/or notch filters, the combination of plug-in modules that must be kept in inventory to permit such selectivity is equal to the multiple of the numbers of different functions desired. In this example, the multiple is (10) (35) (5), which equals 1,750 different plug-in modules that must be kept in inventory. The present inventor recognized that it is costly for a user both to have to purchase 1,750 different plug-ins, or perhaps even a greater number of plug-ins if greater selectivity is required, along with the added cost of maintaining storage space and access to such inventory. Obviously, if a lesser number of different single plug-in modules are retained in inventory, the selectivity capability for different combinations of the aforesaid functions is diminished. Accordingly, there is a long felt need in the art for an improved multi-tap system and devices that permit a user to easily select a combination of different functions for a given multi-tap without requiring an extensive inventory of various components. The present inventor also recognized there will be a significant cost savings if such an improvement is provided on tap plates that can merely be substituted for the tap plates of presently installed multi-taps to avoid the cost of removal and replacement of existing multi-taps.
With the problems of the prior art in mind, in a first embodiment of the invention, a directional coupler is built into the printed circuit (PC) board secured to the inside face of a tap plate or cover plate of a standard multi-tap housing. The PC board also includes an electrical receptacle for receiving an electrical plug-in device for providing a desired equalization of forward and backward or return signals, and/or level of noise filtering, for example. A plurality of such plug-in devices are provided for permitting a user to select the appropriate plug-in device that provides the closest level of a desired equalization, and/or filtering. Also, the tap plates are designed to permit them to directly replace original tap plates not including this first embodiment, or other embodiments of the invention as described below. In this manner, existing cable television systems do not have to replace the entire multi-tap devices, perhaps requiring cable cutting and installation of different connectors.
In a second embodiment of the invention, dual plug-in devices are provided on the PC board of each tap plate. More specifically, a first electrical receptacle is provided on the PC board for receiving one of a plurality of available first plug-in devices for selectively providing a directional coupler having a desired level of attenuation for signals being tapped off of a main distribution cable. Accordingly, unlike the first embodiment, the directional coupling function is provided in the first plug-in device. Note that the direction of signal flow can be reserved by reversing the positioning of the first plug-in device. Each of the available group of individual first plug-in devices provide directional couplers having different levels of attenuation. A second electrical receptacle is provided on the PC board for receiving a desired one of a plurality of second plug-in devices respectively providing different levels of equalization for forward and return signals, which devices may also include noise filtering, and/or other functions.
Other embodiments of the invention provide three or more plug-in devices. A first group of such plug-in devices permit a user to select a desired level of directional coupling, as in the second embodiment of the invention. Second through nth plug-in devices for plugging into associated receptacles on the PC board permit a wide selection of different levels of backward equalization, forward equalization, noise filtering, low pass filtering, high pass filtering, and so forth, respectively.